Updating L51 of common.py

[jackypheno]

Dear Daivd,

I have updated the L51 of flavio/physics/bdecays/common.py, here 'B0': 'bs' is replaced with 'B0': 'bd'.
I request you to update the central repository.

Thanks,
Jacky

[coveralls]

Coverage remained the same at 88.462% when pulling 775e070 on jackypheno:master into 5a90459 on flav-io:master.

[DavidMStraub]

Thanks!